In sedimentary rocks, large vertical changes in properties occur as the result of changes in depositional conditions. Horizontal (or lateral) changes within a formation are known to occur, but these are generally much smaller. However, such changes may be of great importance in petroelum exploration and production. This variation of a property or properties with direction is termed "anisotropy". For example, horizontal anisotropy of elastic properties may affect the interpretation of seismic prospecting data. Petroleum production permeability anisotropy, caused by preferential arrangement of pores and/or fractures, is often an important factor in both primary and enhanced recovery. Stress anisotropy is important in petroleum well stimulation by hydraulic fracturing because the azimuth of induced fractures is generally parallel to the maximum horizontal stress direction. In many cases, the directions of natural and induced fractures are coincident.